As is known, the quality of a fuel, i.e. its chemical and physical characteristics (composition, stoichiometric air/fuel ratio, density and heat value) as a whole, may vary significantly and at times in ways that are unpredictable by the user, and depends on numerous factors, including: different supply sources, different climatic conditions, season-dependent supply factors, different percentages of inert material, different fuel mixtures, etc.
Consequently, in currently marketed vehicles--in which, as is known, the operating injection time of an injector in each operating state of the engine equals a calibration injection time determined at the initial engine calibration stage using a reference fuel, plus a closed-loop injection time correlated to the composition of the exhaust gases produced by the engine--any alteration in fuel quality may affect the behaviour of the engine in terms of performance and emissions and in direct proportion to the extent to which the fuel used differs from the reference fuel used to calibrate the engine.